The invention relates to a process for coating a workpiece with a lubricant based on molybdenum sulfide.
Molybdenum disulfide (MoS2) is a substance similar to graphite which, because of its excellent sliding properties, finds application as a dry lubricant and in composite lubricants. For the most part, molybdenum disulfide is applied in liquid form in a solvent mixture. The layer thickness which remains after drying lies in a range of about 5xcexc to 15xcexc for a tribological requirement. This is suitable for high pressure and temperature requirements and is used especially for improvement of run-in.
It is also known to modify the molybdenum disulfide coating into a combined layer system. Layer systems of this type have been made for TiN and Al2O3: see Bae, Y. W. et al., xe2x80x9cSynthesis and Friction Behavior of Chemically Vapor Deposited Composite Coatings Containing Discrete TiN and MoS2 Phases,xe2x80x9d J. of the American Ceramic Society, 79(4):819-824(1996) and Srivatav, A. et al., xe2x80x9cThe Role of MoS2 in Hard Overlay Coatings of Al2O3 in Dry Sliding,xe2x80x9d Wear, 1955:229-236 (1992).
With the present invention, a long-term usage of lubricants based on molybdenum disulfide is intended. In particular, the adhesion of the lubricant to the workpiece should be decidedly improved, so that the sliding surface formed by molybdenum disulfide has a considerably longer lifetime than before. In this connection, the already known positive properties of this lubricant should remain apparent to the full extent.
This objective is achieved according to the invention in that the workpiece is cleaned and activated, in a manner known per se, prior to coating with molybdenum disulfide, and in that it is then exposed to a galvanic chromium plating bath to form a hard chromium plating having a hardness of at least 600 HV with a beaded (pearl-like) or columnar structured surface, and in that this beaded or columnar structured surface is filled and smoothed by the subsequently applied lubricant based on molybdenum disulfide.
It has been surprisingly revealed that the beaded or columnar structured surface of the hard chromium underlayer exerts an exceptionally high adhesive action on the molybdenum disulfide. In addition to this, to the extent that it extends above the lower hard chromium layer, the molybdenum disulfide is compressed in practical operation within a short time, depending on the use and loading, in a sort of run-in operation, such that the upper ends of the bead or column structure are more or less exposed and function as highly loadable support surfaces for the opposing piece that is to be supported. In contrast, the intermediate spaces, which comprise at least 80% of the contacting surface, are filled with molybdenum disulfide, which results in a lasting reduction of the frictional resistance.
Workpieces coated in this manner have, in tests without additional lubricants, produced a service life up to seventy times greater than workpieces with MoS2 on a smooth hardened surface.
The production of the hard chromium plating with a beaded or columnar structured surface is expediently accomplished according to German Patent 25 02 284. There, a chromium bath is described, with which shiny to metal-gray chromium platings can be obtained with a beaded structured surface having a hardness up to about 1500 HV. These chromium platings with the brand name xe2x80x9cDuralloyxe2x80x9d distinguish themselves by a high wear resistance.
Indeed, from DE 195 29 843 of the same applicant, it is known to coat this hard chromium plating by galvanically applied black chromium, whereby likewise very good sliding properties result. However, one could not generate any excitement from this black chromium coating, instead of providing a non-galvanic coating made of molybdenum sulfide.
In reference to the coating according to the invention with molybdenum disulfide, a galvanic deposition is not suitable. Instead, it is recommended to mix the lubricant with a carrier liquid and to apply it as a liquid, for example, to brush or spray it on. The molybdenum disulfide can thus be present in an air-drying solvent, wherein this solvent contains in particular, esters, butyl acetate, and optionally an inorganic bonding agent. Examples of suitable liquid carrier systems containing MoS2 are known per se and are available commercially, for example from Dow Corning USA under designation nos. DC 7409 and DC 3484. When used on a normal, flat surface, such liquids are rubbed off quickly by normal wear, but on the beaded, pearl-like surface structure of the present invention, the lubricant liquid adheres and remains much longer.
The lubricant should be applied in the process at such a thickness that the remaining molybdenum disulfide after the drying reaches a layer thickness of 5xcexc to 15xcexc.
In order to obtain a good adhesion, it is recommended that the lubricant be applied in gaseous form by vacuum-evaporation or using sputtering technology or gas discharge technology. Besides the so-called physical vapor deposition process (PVD-process), a chemical gas phase deposition (CVD process) also comes into consideration. With these processes either pure molybdenum disulfide is coated orxe2x80x94which is even more expedientxe2x80x94the molybdenum disulfide coating is accomplished together with titanium, wherein the titanium portion amounts at maximum to about 10%. Likewise, the molybdenum disulfide can also be applied together with chromium, nickel, and/or gold.
Also lying within the concept of the invention is to perform the coating by the so-called arc technology: in this process, molybdenum disulfide and other ions are flung out of a source by an electric arc, so that they deposit themselves on the workpiece to be coated.